Bingo. Northern Native Americans were highly nomadic tribes that would follow seasonal herd lines. They did keep bundles of stuff in "caches" but they specialized in setting up temporary abodes that they could easily tear down, roll-up, and pack them.

*Come to think of it, religion plays a big part in "impressive" ruins. Northern Native Americans had deities that came from nature, and paying homage to them was not done through monuments; it was done throughout their daily activities (i.e. a "prayer" [before, after] killing a prey)